Heretofore, there have been known photocatalysts that exhibit activities, by which the decomposition and oxidation of substances are accelerated, when irradiated with light. Recently, attempts or the like have been made to remove air pollutants such as sulfur oxides and nitrogen oxides by utilizing the photocatalysts. Moreover, attempts have been further made to use titanium dioxides as the photocatalysts (see, for example, Japanese Patent Laid-Open Nos. 6-385/1994, 6-49677/1994 and 6-39285/1994 Official Gazettes).
By the way, in recent years, there has been a growing interest in globally environmental pollution. Meanwhile, the demand for removing substances such as CO2, NOx and SOx has grown. Moreover, a plan for creating amenity space by eliminating toxic substances has been devised. Thus, the demands for deodorizing living space and for making the living space antibacterial, soil-resistant and mildew-proof have grown increasingly.
It is accordingly conceived that the aforementioned titanium-dioxide photocatalyst is utilized for removing such pollutants. However, in the case of the conventional titanium dioxide photocatalysts, generally, gaseous or liquid materials to be treated are introduced into a container accommodating the photocatalyst and are thus made to be in contact with the photocatalyst, and simultaneously, light is introduced from the exterior thereto and is applied onto the photocatalyst.
Further, in such a case, for the purpose of increasing the contact area between the material to be treated and the photocatalyst and efficiently applying the light onto the photocatalyst, attempts or the like have been made to produce the photocatalyst in minute-particle form or to hold the photocatalyst on a transparent base material.
However, in the case of the aforementioned conventional titanium dioxide photocatalyst, although the contact area between the photocatalyst and the material to be treated can be increased by, for instance, producing the photocatalyst in minute-particle form, the effective area of the photocatalyst, by which light is received, cannot be increased very much. Consequently, it is difficult to largely enhance the total catalysis effects thereof.
Further, in the case where the titanium dioxide photocatalyst is formed in film form on, for example, a glass substrate or the like, the titanium dioxide photocatalyst itself has low transparency. This is because it has been heretofore considered that methods suitable for forming a photocatalyst in film form to thereby obtain practical photocatalysis are limited to a method of forming a titanium dioxide sol on the substrate by sintering and a method of producing titanium dioxide in fine powder form, dissolving the powder by using a binder and then applying the dissolved powder onto the substrate. However, in the case of employing the former method, a photocatalyst, which has high activity and a certain measure of transparency, can be obtained, though it is necessary for obtaining the film, whose strength is sufficient for practical use, to set a sintering temperature at a value which is not lower than the softening temperature of glass. Thus, at least, it is impossible to form the photocatalyst on the glass substrate. Besides, regarding the light transmissivity, this photocatalyst tends to become clouded. It is difficult for this photocatalyst to transmit visible light to such an extent that the transparency can be obtained. In this sense, this photocatalyst is close to opaque. In contrast, in the case of the latter method, although the step of sintering is unnecessary, the photocatalyst becomes clouded and opaque because fine titanium dioxide powder is applied to the substrate.
Further, in the case of titanium dioxide produced in film form by performing a sol-gel method and CVD method which have been well known in the field of such a kind heretofore, the transparency can be ensured, whereas the activity of the catalyst, which has a practical level, is not obtained.
Thus, all of the conventional titanium dioxide photocatalysts, which exhibit photocatalytic activity on practical levels, are substantially opaque. Therefore, even in the case that this conventional photocatalyst is formed on, for example, the front surface of a transparent glass substrate or the like, light applied from the back surface of the glass substrate cannot effectively reach the front surface portion of the photocatalyst. Consequently, only light applied from the front surface portion, on which the photocatalyst is formed of the substrate can be utilized. Hence, in the case that the cleaning of indoor air is performed by forming this photocatalyst on, for instance, the surface of a window pane, it naturally follows that the photocatalyst is formed on the surface of the glass, which faces the inside of a room. Thus it is only light applied from the inside of the room that can be utilized for obtaining the photocatalyst activity. Consequently, there has been a serious defect that sunlight cannot be utilized therefor.
Thus, in the case of the conventional titanium dioxide photocatalyst, titanium dioxide, which performs the photocatalysts, itself is substantially opaque. Consequently, there occurs a limit to the enhancement of the photocatalytic activity. Moreover, the range of application of the photocatalyst is extremely limited.
Furthermore, there has been made an attempt to apply powdered photocatalyst to the outer surface of a discharge lamp to thereby impart a deodorization function thereto (see Japanese Patent Laid-Open No. 1-169866 Official Gazette). Additionally, there has been made another attempt to cover the periphery of an illuminating lamp with a net constituted by a glass filter, which is coated with a photocatalyst (see Japanese Patent Laid-Open No. 1-139139 Official Gazette), thereby performing a deodorization by utilizing a photocatalytic action at a place where illuminance is high, namely, at a place closer to the illuminating lamp. Besides, there has been made still another attempt to decompose ambient offensive odor (or malodor) substances by depositing a titanium dioxide film on the surfaces of spectacle lenses according to a sputtering method (see Japanese Patent Laid-Open No. 2-223909 Official Gazette).
However, the discharge lamp described in the aforementioned Japanese Patent Laid-Open No. 1-169866 Official Gazette is configured only by applying anatase-type titanium dioxide powder whose grain diameter is 500 Å, onto the outer surface of a discharge container. Thus, this discharge lamp is inferior to other lamps in light transmissivity and abrasion resistance. It is obvious that, even if the applied titanium dioxide is baked, a high temperature is needed and there are obtained only discharge lamps which are inferior to other lamps in light transmissivity. Therefore, in the case of this discharge lamp, the photocatalyst has little effect. Further, this discharge lamp is in a state in which the powder adheres to the surface thereof and the degree of the unevenness of the surface thereof is high. With such a structure, this discharge lamp is easily stained and is liable to gather dust.
Moreover, regarding the air-cleaning spectacle described in the Japanese Patent Laid-Open No. 2-223909 Official Gazette, although the titanium dioxide films are formed on the surfaces of the spectacle lenses by a physical method such as an ion plating method, the objective device configuration and data concerning the identification of titanium dioxide, the crystalline structure of the (thin) films and the judgement on the deodorization effects are not sufficiently disclosed in this official gazette.
Furthermore, in the case that the films are formed by the physical method such as a sputtering method, a considerably long film formation time is required to obtain a film thickness by which practical photocatalytic actions can be caused. This causes problems in respect of the productivity and the stability of the quality of films. Consequently, such physical film formation processes have drawbacks in that such processes are difficult to be used as manufacturing processes of general-purpose industrial products.
Further, the conventional illuminating lamp coated with a titanium dioxide film (or layer) has defects in that the light transmissivity is low because the powdered titanium dioxide film is used and thus this film is substantially opaque, in that it is difficult for light, which is emitted from the inside of the illuminating lamp, to reach the outermost surface of the titanium dioxide layer, to which contaminants in the air most easily adhere, in that therefore, the quantity of available light is considerably smaller than the quantity of available light in the case of depositing a transparent titanium dioxide film on the lamp, in that thus, the amount of decomposed contaminants is much smaller in comparison with the amount thereof in the latter case, and in that the surface of the lamp is easily stained owing to the unevenness of the surface thereof.
Incidentally, in the case of the discharge lamp and the spectacle, which are respectively described in the aforementioned Japanese Patent Laid-Open Nos. 1-169866 and 2-223909 Official Gazettes, objects to be decomposed are mainly offensive odor substances. Namely, the primary objects of purposes of these discharge lamp and spectacle are not the decomposition of fat and oil.
The present invention is accomplished against the aforementioned background. The present invention aims at providing a titanium dioxide photocatalyst structure that has excellent photocatalytic actions and light transmissivity and enables members of various substances, which require transparency particularly, to have photocatalytic actions and further aims at providing a method for producing such a photocatalyst structure.